Down in the right-hand and left-hand corners of these windows is an arm (called a casement stay) that usually, but not always, has a series of holes into which a little bobbin drops to retain the window in an open position. Alternatively an arm winder (turn the handle and the arm moves in an arc) might be used to keep the window open.
Casement stays have not been manufactured for many years. We convert them to arm winders.